Run-flat pneumatic radial tire

ABSTRACT

A run-flat pneumatic radial tire having an aspect ratio of up to 50%, which includes a reinforcing liner layer mode of a rubber having a complex modulus at 20° C. E* 20 , a ratio of a complex modulus at 100° C. to the complex modulus at 20° C., a 100% modulus and a loss tangent (tan δ) at 100° C. which satisfy certain specified numeric values. The liner has a crescent sectional shape and is disposed in sidewall portions of a carcass layer with its end portions overlapping the end portions of a belt layer and a bead filler having a specified hardness and height h from a wheel rim base. The carcass layer consists of an inner carcass layer turned up around the bead core from inside to outside the tire so that its end portion is positioned at a position higher than the height h of the bead filler and an outer carcass layer having an end portion disposed axially outward of the bead core.

BACKGROUND OF THE INVENTION

This invention relates to a pneumatic radial tire having improvedrun-flat durability.

A run-flat pneumatic tire is the tire which can keep running for apredetermined distance even when an internal pressure of the tire dropsdrastically due to puncture and the like. To provide such run-flatperformance, conventional run-flat tires are primarily directed toreduce heat generation at sidewall portions by increasing mainlyrigidity of the sidewall portions as much as possible and making it moredifficult for the tires to undergo flexibility. For example, Japanesepatent application Kokai publication No. 64-30809 proposes a flatrun-flat tire having a reinforcing liner layer which has a crescentsectional shape, is made of a rubber having a JIS--A hardness of 50 to65 and is disposed on the inner surface of the sidewall portions of atire having an aspect ratio of up to 55% so that its upper end overlapswith a belt layer and its lower end overlaps with a bead filler, fromthe aspect that the lower the aspect ratio of the tire, the higher canbe made rigidity and more difficult does it become for the tire toundergo flexibility.

However, since these tires are directed to increase rigidity of thesidewall portions as much as possible so as to reduce their deflection,general excellent running performance inherent to the radial tires arereduced unavoidably. If rigidity of the sidewall portions is extremelyincreased in radial tires having a low aspect ratio, local strainincreases particularly at the portion ranging from near the upper end ofa rim flange to a belt edge of a ground contact portion because theheight of the sidewall portions is small, so that run-flat durabilitydrops.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a run-flatpneumatic radial tire having run-flat durability without deterioratingmarkedly ordinary running characteristics of the radial tires whilemaking the most of the features of a tire having a low aspect ratio inthe aspect of rigidity.

In a run-flat pneumatic radial tire having an aspect ratio of up to 50%,the object of the present invention described above can be accomplishedby a run-flat tire characterized in that a reinforcing liner layer madeof a rubber having a complex modulus at 20° C., E*₂₀, of at least 16 MPaand preferably up to 60 MPa, a ratio of a dynamic elastic modulus at100° C., E*₁₀₀, to the dynamic elastic modulus at 20° C., E*₂₀, that is,E*₁₀₀ /E*₂₀, of at least 0.80 and preferably up to 1.00, a 100% modulusof at least 60 kg/cm² and preferably up to 120 kg/cm², and a losstangent (tan δ) at 100° C. of up to 0.35 and preferably at least 0.05,and having a crescent sectional shape, is disposed inside a carcasslayer at sidewall portions in such a manner that one of its end portionsoverlaps with the end portion of a belt layer of a tread portion and theother end portion overlaps with a bead filler of the bead portion;

the bead filler of the bead portion is made of a rubber having a JIS--Ahardness of from 60 to 80 and has a height h of up to 35 mm from its rimbase in a direction vertical to the axis of rotation of the tire;

The carcass layer consists of inner and outer two layers, the innercarcass layer is turned up from inside to outside the tire around thebead core so that its end is positioned at a position higher than theheight h of the bead filler and is clamped between the inner carcasslayer and the outer carcass layer; and

the outer carcass layer is wound down without being turned up around thebead core in such a manner as to position its end near the bead core, orboth of the two carcass layers are turned up around the bead core frominside to outside the tire so that the end of the carcass layer on theside of the bead core is disposed near the bead core and the end of theother carcass layer is disposed beyond the upper end of the bead filler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are semi-sectional views, showing an example of the tireof the present invention; and

FIGS. 3 and 4 are schematic sectional views, showing the structure ofbead portions of tires which are not the tires of the present invention,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, the tire of the present invention is a flat tirewhich consists of bead portions 1, a pair of right and left sidewallportions 3 continuing from the bead portions 1 and a tread portion 4extending in the tire circumferential direction between these sidewallportions 3, and which has an aspect ratio of below 50% expressed by theratio of the tire sectional height to the tire maximum sectional width.When the aspect ratio is set to be below 50%, rigidity of the sidewallportions of the present tire can be made higher and the tire can be mademore difficult to undergo flexibility than a high aspect ratio tirehaving the sidewall portions made of the same rubber.

A carcass layer 5 consisting of cords crossing substantially at 90° tothe tire circumferential direction has a two-layered structureconsisting of an inner carcass layer 5-1 which is disposed between thebead portions 1 and whose both ends are turned up from inside to outsideof the tire around the bead cores 2 disposed at the bead portions 1 ofthe tire and an outer carcass layer 5-2 which extends down to a positionaxially outside the bead cores 2. Each end of the inner carcass layer5-1 turned up around the bead core 2 is clamped between a portion of thecarcass layer 5-1 before the inner carcass layer is turned up and theouter carcass layer 5-2. Two belt layers 6 which have a cord angle of10°˜30° relative to the tire circumferential direction and cross oneanother are disposed on the carcass layer 5 at the tread portion 4 and acover layer 7 is disposed further on this belt layer 6.

A reinforcing liner layer 8 whose upper end in the radial directionoverlaps with the lower end of the belt layer 6 and whose lower end inthe radial direction overlaps with a bead filler 9 and which has acrescent sectional shape is disposed inside the carcass layer 5 of eachsidewall portion 3. The bead filler 9 is disposed above the bead core 2and wrapped in by the main body of the inner carcass layer 5-1 and theinner carcass layer 5-1 which is turned up from inside to outside thetire around the bead core 2.

In the present invention, the reinforcing liner layer to be disposedinside each sidewall portion must have a crescent sectional shape sothat the thickness is the greatest at the center and decreases graduallytowards the upper and lower portions in the radial direction of thetire, respectively. Moreover, this crescent sectional shape must be suchthat one of its ends overlaps with the end portion of the belt layer andthe other end overlaps with the bead filler of each bead portion. Whenthe reinforcing liner layer has such a sectional shape, the sidewallportion undergoes smooth deformation, prevents the occurrence of localstrain and can limit the drop of general running performance of a radialtire because suitable flexibility is permitted.

The rubber constituting the crescent reinforcing liner layer describedabove must have a complex modulus E*₂₀ at 20° C. of at least 16 MPa.For, if the complex modulus E*₂₀ is lower than 16 MPa under thecondition of the aspect ratio of below 50%, the local strain of thereinforcing liner layer is increased. The rubber described above mustkeep this standard not only at 20° C. but also when the temperaturerises due to running. Therefore, the ratio of the complex modulus E*₁₀₀at 100° C. to E*₂₀, that is, E₁₀₀ /E*₂₀, of this rubber must be at least0.80. Furthermore, this rubber must have a 100% modulus of at least 60kg/cm². If the 100% modulus is less than 60 kg/cm², distortion of thetire sidewall portions becomes great as a whole at the time of run-flatrunning and destruction of the tire is promoted.

Moreover, tan δ of this rubber at 100° C. must be below 0.35. If anyrubber having tan δ exceeding this value is used, heat generation at thereinforcing liner layer becomes so great at the time of bending thatdeflection of the side portions becomes great and run-flat durabilitydrops.

In the present invention, the complex modulus at 20° C. E*₂₀, thecomplex modulus at 100° C. E*₁₀₀ and tan δ at 100° C. are measured underthe condition of a frequency of 20 Hz, an initial strain of 5% and adynamic strain 1% elongation by use of a viscoelastic spectromer"Rheograph Solid" produced by Toyo Seiki K. K., respectively. The 100%modulus is measured by the measurement method stipulated in JIS K 6301.

If the thickness of the reinforcing liner layer having the crescentsectional shape is small, the effect of improving run-flat performancecannot be obtained. Therefore, this thickness is generally made greaterwith an increasing weight of a vehicle. If this thickness is too great,rigidity becomes too great, and general running characteristics as aradial tire will drop. It is therefore advisable that the maximumthickness of this reinforcing liner layer be from 3 mm to 8 mm.

In the tire of the present invention, the bead filler is made of arubber having a JIS--A hardness of 60 to 80 which is lower than thehardness of the reinforcing liner layer, and its height h from a rimbase in a direction vertical to the axis of rotation of the tire must besmaller than 35 mm. If the hardness of the bead filler is greater thanJIS--A hardness of 80 or if the height h is greater than 35 mm, thelocal strain of the sidewall portions is increased and smoothdeformation becomes difficult to occur. If the JIS--A hardness of thebead filler is lower than 60, general running performance of the radialtire such as maneuvering stability will drop. In the present invention,the JIS--A hardness described above is the value measured by themeasurement method stipulated in JIS K 6301.

In the tire of the present invention, the structure of the carcass layeraround the beads becomes another important factor in order to insure thesmooth deformation to reduce the local strain while imparting suitablerigidity to the sidewall portions and permitting small flexibility. Asshown in FIG. 1, in the two-layered carcass layer, the inner carcasslayer 5-1 is turned up around the bead core 2 from inside to outside thetire in such a manner as to position its end to a position higher thanthe height h of the bead filler 9 and to be clamped by the outer carcasslayer 5-2 extending down to a position axially outside the bead core 2.Alternatively, as shown in FIG. 2, two carcass layers 5-1a and 5-1b areturned up around the bead core 2 from inside to outside the tire, theend of one of the carcass layers 51-b is disposed near the bead core 2and the end of the other carcass layer 5-1a is turned up beyond theupper end of the bead filler 9. Among these structures, the tire havingthe carcass layer of the structure shown in FIG. 1 is superior from theaspect of run-flat durability. These carcass layer structures exhibitthe synergistic effects in improving flexibility resistance and limitingthe local strain when combined with the reinforcing liner layer and makesmooth deformation possible, so that run-flat durability can be improvedremarkably.

In the tire of the present invention, known tire retaining mechanisms,such as the bead shape and the rim structure are naturally employed inorder to prevent rolling-off of the tire from the rim even when the airpressure of the tire drops.

Hereinafter, effectiveness of each of the constituent requirements ofthe present invention described above will be explained with referenceto Examples.

In each of the following Examples, the term "run-flat durability"represents the value obtained by measuring the distance until the tiresget out of order when the test tires are fitted to a vehicle receiving aload of 500 kg per tire and so as to prevent the test tires from rollingoff from the rims at an air pressure of 0 kg/cm².

In each Example, run-flat durability is represented by an index and thegreater this index value, the higher run-flat durability.

EXAMPLE 1

Seven kinds of tires A, B, C, D, E, F and G are produced and theirrun-flat durability is evaluated. Each tire is a radial tire having thestructure shown in FIG. 1, the following tire specification and a rubberblend composition (Table 1) for the bead filler in common with theothers but having different rubber characteristics only for thereinforcing liner layer, as tabulated in Table 2. The size of each tireis 255/40R17.

Incidentally, run-flat durability is an index using the evaluationresult of the tire B as 100.

belt cover layer=nylon cord having cord angle of 0° with respect to thetire circumferential direction

belt layer=steel belt having a crossing angle of 24°

carcass layer=rayon cords

JIS--A hardness of bead filler=75, height h=33 mm

maximum thickness of reinforcing liner layer=5 mm

aspect ratio=40%

                  TABLE 1                                                         ______________________________________                                                        Blend amount                                                  blend component (parts per hundred rubber)                                    ______________________________________                                        natural rubber (SIR 20)                                                                       70.00                                                         SBR "Nipol" 1502*                                                                             30.00                                                         zinc oxide      5.00                                                          stearic acid    2.00                                                          anti-oxidant    1.00                                                          carbon black HAF                                                                              70.00                                                         aromatic oil    7.00                                                          accelerator     1.00                                                          sulfur          3.00                                                          ______________________________________                                         *A tradename for a low temperature polymerization styrenebutadiene rubber     (SBR) manufactured by Nippon Zeon K.K., a corporation of Japan.          

                                      TABLE 2                                     __________________________________________________________________________               Tire A                                                                            Tire B                                                                            Tire C                                                                            Tire D                                                                            Tire E                                                                            Tire F                                                                            Tire G                                                Comp.                                                                             Inven-                                                                            Inven-                                                                            Comp.                                                                             Comp.                                                                             Comp.                                                                             Inven-                                                Ex. tion                                                                              tion                                                                              Ex. Ex. Ex. tion                                       __________________________________________________________________________    Blend                                                                         Composition                                                                   natural rubber                                                                           40.00                                                                             40.00                                                                             40.00                                                                             100.00                                                                            40.00                                                                             40.00                                                                             40.00                                      TSR*.sup.1                                                                    UBEPOL VCR 412*.sup.2                                                                    --  --  10.00                                                                             --  --  --  30.00                                      "Nipol" BR 1220*.sup.3                                                                   60.00                                                                             60.00                                                                             50.00                                                                             --  60.00                                                                             60.00                                                                             30.00                                      Zinc oxide 5.00                                                                              5.00                                                                              5.00                                                                              5.00                                                                              5.00                                                                              5.00                                                                              5.00                                       stearic acid                                                                             1.50                                                                              1.50                                                                              1.50                                                                              1.50                                                                              1.50                                                                              1.50                                                                              1.50                                       anti-oxidant 6C                                                                          1.00                                                                              1.00                                                                              1.00                                                                              1.00                                                                              1.00                                                                              1.00                                                                              1.00                                       resin*.sup.4                                                                             1.00                                                                              8.00                                                                              8.00                                                                              1.00                                                                              5.00                                                                              8.00                                                                              8.00                                       KOMOREX 300*.sup.5                                                                       2.00                                                                              --  --  1.00                                                                              --  --  --                                         carbon black                                                                             60.00                                                                             65.00                                                                             65.00                                                                             60.00                                                                             60.00                                                                             --  65.00                                      LS-HAF                                                                        carbon black ISAF                                                                        --  --  --  --  --  65.00                                                                             --                                         insoluble sulfur                                                                          8.40                                                                             7.50                                                                              7.50                                                                              7.50                                                                              8.40                                                                              8.40                                                                              8.40                                       accelerator*.sup.6                                                                       1.20                                                                              1.20                                                                              2.50                                                                              1.50                                                                              1.20                                                                              1.20                                                                              2.50                                       Properties                                                                    E*.sub.20  12.5                                                                              17.4                                                                              23.5                                                                              9.6 18.0                                                                              20.2                                                                              30.2                                       E*.sub.100 10.3                                                                              14.7                                                                              21.2                                                                              6.9 16.0                                                                              18.9                                                                              28.3                                       E*.sub.100 /E*.sub.20                                                                    0.82                                                                              0.86                                                                              0.90                                                                              0.71                                                                              0.89                                                                              0.94                                                                              0.94                                       100% modulus                                                                             56  65  80  59  56  78  92                                         tan δ                                                                              0.10                                                                              0.28                                                                              0.33                                                                              0.15                                                                              0.16                                                                              0.38                                                                              0.30                                       run-flat durability                                                                      62  100 108 56  85  93  115                                        __________________________________________________________________________     *.sup.1 Technically Specified Rubber;                                         *.sup.2 a tradename for syndiotactic polybutadiene manufactured by UBE        Kosan K.K., a corporation of Japan;                                           *.sup.3 "Nipol" BR 1220 is a tradename for polybutadiene rubber               manufactured by Nippon Zeon K.K., a corporation of Japan;                     *.sup.4 methacresolformaldehyde resin;                                        *.sup.5 aromatic oil extender 20% manufactured by Nippon Oil Co., Ltd., a     corporation of Japan; and                                                     *.sup.6 Noxydiethylene-2-benzothiazolyl sulfenamide (OBS).               

It can be understood from Table 2 that run-flat durability drops if anyone of E*₂₀, ratio E*₁₀₀ /E*₂₀, 100% modulus and tan δ of thereinforcing liner layer is out of the range stipulated in the presentinvention.

EXAMPLE 2

Run-flat durability is evaluated for eight kinds of tires B and Hthrough N, shown in Table 3, produced by changing the bead filler heighth (mm) and its JIS--A hardness of the tire B used in Example 1. In thetable, run-flat durability is expressed by an index using the evaluationresult of the tire E as 100.

                  TABLE 3                                                         ______________________________________                                                         Bead filler                                                                   Height                                                                              JIS-A    Run-flat                                                       (h)   hardness durability                                    ______________________________________                                        Tire B (This Invention)                                                                          33 mm   75       211                                       Tire H (Comparative Example)                                                                     43 mm   90       100                                       Tire I (Comparative Example)                                                                     43 mm   75       102                                       Tire J (Comparative Example)                                                                     38 mm   90       103                                       Tire K (Comparative Example)                                                                     38 mm   75       106                                       Tire L (Comparative Example)                                                                     33 mm   90       132                                       Tire M (Comparative Example)                                                                     28 mm   90       157                                       Tire N (This Invention)                                                                          28 mm   75       208                                       ______________________________________                                    

It can be understood from Table 3 that if the height h of the beadfiller from the rim base is greater than 35 mm, run-flat durability isnot improved (tires H through K). In contrast, run-flat hardness can beimproved in all of the Tires B, L, M and N wherein the height h is below35 mm. However, the degree of improvement is low in the tires having ahigh JIS--A hardness and run-flat durability can be further improved byreducing this hardness (tires B and N).

The reason is believed as follows: In addition to the disposition of thereinforcing liner layer having the crescent sectional shape describedalready, the local strain of the sidewall portions, particularly nearthe portions from the upper end of the rim flange to the belt edge ofthe ground contact portion, can be reduced and the smooth deformationcan take place by reducing the height of the bead filler and decreasingits hardness.

EXAMPLE 3

Run-flat durability is evaluated for three kinds of tires O, P and Qproduced by changing only the structure near the beads of the carcasslayer of the tire B of Example 1, as shown in FIGS. 2, 3 and 4, and forthe tire B. The results are tabulated in Table 4. In this table,run-flat durability is expressed by an index using the evaluation resultof the tire B as 100.

In the bead portion shown in FIG. 3, both of the two carcass layers 5-1aand 5-1b are turned up around the bead core 2 and the end portions ofthese carcass layers 5-1a and 5-1b are disposed at positions higher thanthe height h of the bead filler 9. In the structure shown in FIG. 3, thebead portion shown in FIG. 4 has the structure wherein the end portionsof the two carcass layers 5-1a and 5-1b are disposed at positions lowerthan the height h of the bead filler 9. Furthermore, in the bead portionshown in FIG. 2, both the two carcass layers 5-1a and 5-1b are turned uparound the bead core 2, the end portion of one 5-1a of the carcasslayers is disposed near the bead core 2 and the end portion of the othercarcass layer 5-1b is disposed at a position higher than the height h ofthe bead filler 9.

                  TABLE 4                                                         ______________________________________                                        Tire B       Tire O      Tire P     Tire Q                                    This         Comparative Comparative                                                                              This                                      Invention    Example     Example    Invention                                 ______________________________________                                        Structure                                                                             FIG. 1   FIG. 3      FIG. 4   FIG. 2                                  around                                                                        bead                                                                          Run-flat                                                                              100      63          54       89                                      durability                                                                    ______________________________________                                    

As can be understood from the table, Tires 0 and P have remarkably lowrun-flat durability. Though run-flat durability of the Tire Q is lowerthan that of the tire B, it is not lower than that of the tires 0 and Pand the Tire Q has practically usable run-flat durability.

What is claimed is:
 1. In a radial tire having an aspect ratio of up to50%, a run-flat pneumatic radial tire characterized in that:areinforcing liner layer made of a rubber having a complex modulus at 20°C., (E*₂₀,) of at least 16 MPa, a ratio of a complex modulus at 100° C.,(E*₁₀₀,) to said complex modulus E*₂₀, (E*₁₀₀ /E*₂₀) of at least 0.80, a100% modulus of at least 60 kg/cm² and a loss tangent (tan δ) at 100° C.of up to 0.35, said E*₂₀, E*₁₀₀, and loss tangent at 100° C. beingmeasured under conditions of a frequency of 20 Hz, an initial strain of5% and a dynamic strain of 1% elongation, and having a crescentsectional shape is disposed inside a carcass layer at sidewall portionsin such a manner that one of its end portions overlaps with the endportion of a belt layer of a tread portion and the other end portionoverlaps with a bead filler of said bead portion; said bead filler ofsaid bead portion being made of a rubber having a JIS--A hardness offrom 60 to 80 and has a height h of up to 35 mm from a rim base of saidbead portion in a direction vertical to the axis of rotation of saidtire; and said carcass layer consisting of inner and outer two layers,said inner carcass layer is turned up from inside to outside said tirearound said bead core so that its end is positioned at a position higherthan the height h of said bead filler and is clamped between said innercarcass layer and said outer carcass layer and an end of said outercarcass layer is disposed axially outward of said bead core withoutbeing turned up around said bead core.
 2. A run-flat pneumatic radialtire according to claim 1, wherein the thickness of said reinforcingliner layer having a crescent sectional shape is the greatest at thecenter portion and decreases gradually towards the upper and lowerportions in the radial direction of said tire.
 3. A run-flat pneumaticradial tire according to claim 2, wherein the maximum thickness of saidreinforcing liner layer at the center portion is from 3 mm to 8 mm. 4.In a radial tire having an aspect ratio of up to 50%, a run-flatpneumatic radial tire characterized in that:a reinforcing liner layermade of a rubber composition containing a natural rubber, apolybutadiene rubber and a syndiotactic polybutadiene rubber and havinga complex modulus at 20° C., (E*₂₀,) of at least 16 MPa, a ratio of acomplex modulus at 100° C. (E*₁₀₀,) to said complex modulus E*₂₀, (E*₁₀₀/E*₂₀) of at least 0.80, a 100% modulus of at least 60kg/cm² and a losstangent (tan δ) at 100° C. of up to 0.35, said E*₂₀, E*₁₀₀, and losstangent at 100° C. being measured under conditions of a frequency of 20Hz, an initial strain of 5% and a dynamic strain of 1% elongation, andhaving a crescent sectional shape is disposed inside a carcass layer atsidewall portions in such a manner that one of its end portions overlapswith the end portion of a belt layer of a tread portion and the otherend portion overlaps with a bead filler of said bead portion; said beadfiller of said bead portion being made of a rubber having a JIS--Ahardness of from 60 to 80 and has a height h of up to 35 mm from a rimbase of said bead portion in a direction vertical to the axis ofrotation of said tire; and said carcass layer consisting of inner andouter two layers, said inner carcass layer is turned up from inside tooutside said tire around said bead core so that its end is positioned ata position higher than the height h of said bead filler and an end ofsaid outer carcass layer is disposed axially outward of said bead core.5. A run-flat pneumatic radial tire according to claim 4, wherein thethickness of said reinforcing liner layer having a crescent sectionalshape is the greatest at the center portion and decreases graduallytowards the upper and lower portions in the radial direction of saidtire.
 6. A run-flat pneumatic radial tire according to claim 5, whereinthe maximum thickness of said reinforcing liner layer at the centerportion is from 3 mm to 8 mm.